The invention relates to prostheses or implants having a surface modification which improves the process of osteointegration. The invention also relates to methods for making such prostheses or implants and prosthetic or implant kits.
A major unresolved clinical problem in the management of orthopaedic conditions is the ability to implant orthopaedic prostheses, which achieve permanent fixation to surrounding bone. Presently, metal devices, whether cemented or non-cemented, show only poor osteointegration with a finite lifetime before loosening; porous ceramic coatings provide improvements but do not offer complete resolution1. There is considerable scope for improvement of orthopaedic implants, particularly in development of uncemented devices, which aim to improve osteointegration between implant and bone.
Cemented prostheses suffer from problems associated with thermal and chemical bone necrosis, cement shrinkage and stiffness mismatch, weak-link zones at {bone:cement:implant} interfaces and cement particles causing inflammation and bone erosion2,3,4. Uncemented prostheses were developed to overcome the problems associated with cement, however the clinical outcomes of these systems have been below expectation; the main problems being associated with failure of the bone:implant interface and osteolysis (in common with cemented systems)5.
The outcome of surgery to install the implant is heavily dependent upon how the implant interacts with the host both in acute and chronic phases of healing. During the acute phase the inflammatory response is directly related to the surgical intervention. However the implant characteristics and its proximity to the bone come into play, fundamentally influencing the degree of osteointegration and therefore the longevity of the implant. Although not fully understood, the implant surface is thought to play an important role in osteointegration. Therefore the positioning of a biomaterial with a physical ultra-structure capable of forming a matrix or a scaffold for osteogenic cell attachment between the {bone:implant} interface may be advantageous in promoting osteointegration. Critical factors influencing the success of such biomaterials include biocompatibility, cellular adhesion, physical ultra-structure, and degradation,(related to residence time of the system).
Metals (e.g. titanium), ceramics (e.g. hydroxyapatite, bioglasses), and polymers (e.g. polyethylene oxide) are the biomaterials most frequently used as prosthetic alternatives to natural bone. These materials may be considered to be osteoconductive since they appear to offer acceptable support for cell attachment, growth and vascularisation. Beyond osteoconductivity, however, the principal properties demanded of these materials are mechanical strength and osteointegration. In particular, osteointegration, defined as a “direct structural and functional connection between ordered living bone and the surface of a load-carrying implant”, is the major characteristic which ensures good long-term prosthesis functionality. In the case of titanium, hydroxyapatite and bioactive glass implants, mineralised bone is rarely deposited closer than 100 to 500 nm from the material surface; ultrastructural examination reveals an electron-dense zone interspersed between the mineralised tissue of the regenerating bone and the material surface. In this region, an interfacial layer of randomly distributed collagen filaments and proteoglycan exists; matrix mineralisation only takes place at the level of ordered collagen bundles.
Previous reports in the literature have reported the use of liposomal systems to investigate the role of matrix vesicles in bone growth. These matrix vesicles are thought to be the initial site of calcium phosphate precipitation in vitro6. Liposomes composed of phosphatidylcholine: dicetylphosphate: cholesterol (7:2:1 molar ratio) and an ionophore were used to demonstrate the transport of calcium into the liposomes and the formation of hydroxyapatite7,8. It was also demonstrated that no calcium phosphate was produced in these liposomes in the absence of an ionophore9. A decrease in the amount of free calcium in the buffer of approximately 0.2 mM or 9% of the initial calcium concentration within a 6-hour time period in liposomes containing dicetylphosphate was reported7,10.
There is no evidence in the literature of the association of phospholipids with surfaces with the intention of precipitating calcium phosphate onto the surface. European Patent Number EP 0806212 refers to a technology to precipitate calcium phosphate onto the surface of an implantable device and lists the co-precipitation of biologically active substances onto the surface during the manufacture of the coating. No mention is given for the co-precipitation of calcium phosphate and phospholipids, no information is supplied as to their function and no contribution is claimed for an increase in the rate of precipitation. Japanese patent number JP 3294221 refers to the coating of ceramics with phospholipids and drug molecules. The phospholipids form liposomes containing the drug and appear to be entrapped within the holes in the ceramics. The stated purpose of the phospholipids is to prevent the infection of implanted artificial teeth by acting as a depot or slow release system for the drug molecule and no claim appears to be made for improving osteointegration of said artificial teeth. European Patent number EP0479582 refers to the use of antibiotic-containing liposomes combined with hydroxyapatite and collagen and placed into the area of resorbed jawbone to generate new bone tissue. No other claims are made with regards to any properties attributed to the presence of the phospholipids other than as carriers for the antibiotic. U.S. Pat. No. 5,755,788 describes the binding of liposomes to the surface of prostheses and implants which are designed to resist thrombosis development in the body.
It is an object of the invention to provide an improved prosthesis or implant which is susceptible of improved osteointegration or implant in vivo.